I'd run it by "states." Have the cars check things around them and do stuff accordingly. For example, when a car comes to a 4 way intersection, have it choose a random direction. Or you could block out choices where there's a road block set up. This would run a LOT of private variables and global variables. For regular cars you wouldn't need as many variables, but they'd work on the same principles. If you are driving a car, you could use the Car Object which should be more "car-like" but that won't work for AI. For the AI I'd use Custom Movement and work on tell it to detect when there's a turn or a solid in front of it. You could check out Line of Sight too.

Basically, AI can get complex. I hope this gave you some ideas at least. Maybe someone who knows more will come along and give you an example, but I haven't delved into this sort of thing myself yet so I'm just throwing ideas out there.